LTE is a particular standard in mobile telecommunications network technology. The current generation of mobile networks is collectively known as 3G (for “third generation”) and generally employs Wideband Code Division Multiple Access (W-CDMA) technology. LTE is a 4th generation standard of radio technologies designed to increase the capacity and speed of mobile telephone networks and is often marketed as 4G. In the uplink, LTE uses a pre-coded version of orthogonal frequency-division multiplexing (OFDM) called Single Carrier Frequency Division Multiple Access (SC-FDMA). In the uplink, there are typically three physical channels.
The physical channels in mobile radio systems are usually multipath fading channels, which can cause inter channel interference in the received signal. To remove this interference from the signal, a variety of equalizers can be used based on detection algorithms. These detectors require knowledge of the channel impulse response (CIR), which can be provided by a separate channel estimator. Usually, channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples.
Within LTE, coherent detection can be used to improve the detection and decoding processes at the receiver. In order to facilitate a coherent detection process, the receiver needs to estimate the current channel conditions which may be time varying and which may also depend upon the propagation environment. Typically, pilot symbols, whose data information is known to the receiver, are inserted into the signal at the transmitter. Alternatively, the transmitter may insert pilot symbols periodically, using the individual channelization codes of each multiplexed signal; thus each signal has its own time-multiplexed pilot symbols inserted in between data symbols. At the receiver, changes in the channel conditions are interpolated using appropriate algorithms.
In radio technologies such as LTE, multiple-input and multiple-output (MIMO) is the use of multiple antennas at both the transmitter and receiver to improve communication performance and to exploit the availability of multiple independent radio terminals in order to enhance the communication capabilities of each individual terminal. Multi-user MIMO (MU-MIMO) technology is a likely candidate technology for adoption by the industry since MU-MIMO is more feasible to low complexity mobiles with small number of reception antennas than single user MIMO (SU-MIMO).
Thus, there is a need for flexible and improved channel estimation in LTE for the uplink SC-FDMA when MU-MIMO is used.